1. Field of Invention
This invention relates generally to electric motor control circuits. More specifically, the invention relates to electric motor control circuits which set an upper limit on the rotation speed of electric motors and adjust the rotation speed within the set upper limit.
2. Description of Related Art
It is known to provide an electric motor control circuit which sets an upper limit on the rotation speed of an electric motor and adjusts the rotation speed within the upper limit. In this regard see U.S. Pat. No. 4,734,629, the teachings of which are incorporated herein by reference as if fully set forth, and the concept of which is explained in FIGS. 6 and 7.
FIG. 6 is a schematic diagram of the configuration of a known electric motor control circuit. FIG. 7 is a schematic diagram of a part of the electric motor control circuit shown in FIG. 6.
This known electric motor control circuit has a triac Q7 for controlling a voltage applied to motor M from an interchange power supply 50. A transistor Q6 provides gate pulse signals to triac Q7. A tachometer-generator TG detects the rotation speed of the motor M. An upper limit rotation speed setter S10 (see FIG. 7) sets the upper limit of rotation speed of the motor M. A trigger switch S9 adjusts the rotation speed of the motor M within a range having the upper limit. A phase control IC 60 controls a gate pulse signal outputting timing from transistor Q6.
The signal indicative of rotation speed of the motor M detected by tachometer-generator TG is input to a terminal P30, and converted into a voltage signal corresponding to the rotation speed of the motor M via a frequency/voltage converter 61 (refer to FIG. 7). A signal indicative of the upper limit of rotation speed output from setter S10 is input to a terminal P31. The voltage of both signals are compared by an operational amplifier 62 built in phase control IC 60, and a pulse signal indicative of this comparative result is input to a pulse timing setting circuit 63. A sawtooth signal which synchronizes with the interchange power supply 50 is input to the pulse timing setting circuit 63 via a terminal P41, and an agreement point of the voltage of this sawtooth signal and the voltage of the pulse signal is detected.
At the detected agreement point, an operation signal is output from the pulse timing setting circuit 63 to transistor Q6 via terminal P39. A gate pulse signal (trigger current) is output to triac Q7 from transistor Q6, and triac Q7 turns on, and the phase of the voltage applied to motor M is controlled.
Phase control IC 60 watches the rotation speed of motor M and controls the phase of the voltage applied to motor M to maintain the upper limit rotation speed set by setter S10.
The output of operational amplifier 62 is connected to trigger switch S9 via a terminal P34 and a transistor Q5. The voltage level of the output signal of operational amplifier 62 is changed by operating this trigger switch S9.
By operating trigger switch S9, the output timing of the pulse signal in the pulse timing setting circuit 63 is adjusted, and, the rotation speed of the motor M is adjusted within the range that the rotation speed doesn't exceed the upper limit rotation number.
In this known electric motor control circuit, however, trigger switch S9 is connected to the output side of the operational amplifier 62 as shown in FIG. 7. The output level of the operational amplifier 62 is adjusted directly by the trigger switch S9. Accordingly, even if the rotation speed of the motor M decreases due to a load and the decreased number is detected by operational amplifier 62, the rotation speed of the motor M can't be increased because the output of operational amplifier 62 is lowered by trigger switch S9.
That is, as shown in FIG. 8 showing a relationship between a load applied to motor M and the rotation speed of motor M controlled by the existing control circuit, if trigger switch S9 is set to the maximum stroke and the rotation speed reaches the upper limit rotation number, the rotation speed of motor M is not slowed by a certain load because the output level of operational amplifier 62 does not increase. Whereas the rotation speed is slowed by a load until the rotation speed reaches the upper limit rotation.
Specially, in an electromotive tool such as a sander and a polisher, because required rotation speeds of the motor are different due to finishing stages and polished parts, the rotation speed of the motor is changed frequently. In the above mentioned existing circuit, unless the stroke of the trigger is made longer, the rotation speed of the motor falls when a pad for grinding is put on a grinding side. When the rotation speed of the motor is changed, an established value of the upper limit rotation speed setter S10 must be adjusted, which is inconvenient. Moreover, it is desirable that the rotation speed of the motor can be changed by adjusting the stroke of the trigger, because delicate finishing is carried out with delicately changing the rotation speed of the motor in the grinding operation.